1. Field of the Invention
The present invention relates generally to wireless mobile telecommunication networks and, more specifically, to the handoff or handover of a mobile station from one cell to another.
2. Description of the Related Art
“Handoff” or “handover” is a term that refers to the process or method by which a cellular telecommunication network passes the task of serving a mobile telephone handset, wireless data device, or other such mobile station (MS) from one base station radio transceiver to another. Handover is generally performed for one of two reasons. The first and most common reason is to maintain a user operating a MS in wireless (radio) communication as the user moves from one geographic area served by the network to another. A cellular communication network comprises numerous adjacent cells, each of which is defined by a base station or Base Transceiver Station (BTS) that can serve, i.e., communicate with, any active MS within a certain reception range or range within which good signal quality and strength can be expected. The cells are thus roughly circular in shape, with their diameters defined by this range, and generally overlap adjacent cells to some extent. Nevertheless, for convenience, cells are typically graphically represented on geographic network maps as interconnected polygons, most commonly hexagons. As the MS moves away from the BTS, i.e., toward the cell edge or boundary, the signals communicated between the MS and the BTS fade or otherwise degrade. One or more network entities monitor signal quality, strength or similar measurement of how “good” a signal is between the MS and each of the various cells in the vicinity of the MS. The measured quantities are compared with one another to identify the cell with which the MS communicates the highest quality or otherwise “best” signals. If it is determined that another cell would communicate better signals than the cell currently serving the MS, the MS is handed over from the then-serving cell to the other cell. That is, the cell to which the network hands over the MS begins serving the MS, and the cell from which the network hands over the MS ceases to serve the MS. Such a handover may occur again from time to time as the MS moves about.
Another reason for performing handover is to balance the load among the BTS's so that one BTS is not overloaded with calls while another that is capable of handling one or more of those calls remains underutilized. Some cellular networks are configured to hand over a MS from the serving BTS to another BTS when the serving BTS is serving many more MS's than the other BTS.
The network element that monitors the signals and is primarily responsible for the decision whether to hand over a MS to a different cell depends upon the network type (e.g., GSM, UMTS, CDMA, etc.), but in many networks the element is known as a Base Station Controller. An analogous network entity is known as a Radio Network Controller (RNC) in the context of other types of networks. For purposes of this patent specification, the term “Base Station Controller” (BSC) includes within its scope not only that which is commonly referred to as a BSC but also that which is commonly referred to as an RNC and all such other analogous network elements. Likewise, for purposes of this patent specification, the terms “Base Transceiver Station” (BTS) and “Base Station” (BS) are intended to include not only network elements that are commonly referred to by those terms but also other such elements that serve analogous functions in the context of other cellular network types. The BSC includes processing logic that performs an algorithm involving the above-mentioned signal comparison. Various handover algorithms are known in the art. One well-known example of such an algorithm is known as Mobile Assisted Handoff (MAHO). In the MAHO algorithm, signal strength and quality of the voice signals the MS is receiving from its serving cell, plus the control signals of neighboring cells, are compared with each other, in part by the UE itself, to determine the best cell to serve the MS.
A successful handover involves a coordinated process in which the serving BTS relinquishes control over the UE, and the BTS to which the MS is handed over takes control and establishes communication with the MS in an essentially seamless manner, such that a call in progress can continue uninterrupted. Unsuccessful handover can result in a dropped call, i.e., a situation in which the call is permanently disrupted because no communication link relating to the call remains between the MS and any BTS.
“Emergency handover” or “rescue handover” is a term that has been applied to the situation in which the BSC or other handover decision-making entity determines that a call must be handed over immediately in order to avoid it being dropped. Although handover decisions are most commonly based upon signal quality measurements, a rescue handover decision is typically based upon signal strength alone. That is, if the BSC determines that the strength of signals received from the MS have fallen below a threshold necessary to maintain the communication link, the BSC initiates a rescue handover from the serving BTS to any other BTS that is capable of handling the call, even though conditions do not otherwise indicate that “normal” handover is to occur.
A situation can occasionally occur when a BSC determines that rescue handover should occur but there is no BTS with available capacity to handle the call, i.e., all candidate BTS's are fully loaded handling other calls. Such a situation often results in a dropped call.
It would be desirable to enhance existing handover methods so that fewer calls are dropped. The present invention addresses this problem and others in the manner described below.